This invention relates to an improved system and method for measuring the diameter of silicon crystals grown by the Czochralski process and, in particular, a system or method for use in controlling an apparatus or method employing the Czochralski process.
The substantial majority of monocrystalline silicon used to make silicon wafers for the microelectronics industry is produced by crystal pulling machines employing the Czochralski process. Briefly described, the Czochralski process involves melting chunks of high-purity polycrystalline silicon in a quartz crucible located in a specifically designed furnace to form a silicon melt. A relatively small seed crystal is mounted above the crucible on the lower end of a pull wire hanging from a crystal lifting mechanism for raising and lowering the seed crystal. The crystal lifting mechanism lowers the seed crystal into contact with the molten silicon in the crucible. When the seed begins to melt, the mechanism slowly withdraws it from the silicon melt. As the seed is withdrawn, it grows drawing silicon from the melt. During the growth process, the crucible is rotated in one direction and the crystal lifting mechanism, wire, seed, and crystal are rotated in an opposite direction.
As crystal growth is initiated, the thermal shock of contacting the seed with the melt may cause dislocations in the crystal. The dislocations are propagated throughout the growing crystal and multiplied unless they are eliminated in the neck region between the seed crystal and the main body of the crystal. The known methods of eliminating dislocations within silicon single crystal involve growing a neck having a small diameter at a relatively high crystal pull rate to completely eliminate dislocations before growing the body of the crystal. After dislocations are eliminated in the neck, its diameter is enlarged until the desired diameter of the main crystal body is reached. When the neck, which is the weakest part of the crystal, has too small of a diameter, it can fracture during crystal growth, causing the body of the crystal to drop into the crucible. The impact of the crystal ingot and splashing molten silicon can cause damage to the crystal growing apparatus as well as present a serious safety hazard.
As is known in the art, the Czochralski process is controlled, in part, as a function of the diameter of the crystal being grown. Thus, for both control and safety reasons, an accurate and reliable system for measuring crystal diameter, including neck diameter, is needed.
Several technologies are known for providing crystal diameter measurements including methods of measuring the width of the bright ring. The bright ring is a characteristic of the reflection of the crucible wall in the meniscus which is formed at the solid-liquid interface. Conventional bright ring and meniscus sensors employ optical pyrometers, photocells, rotating mirrors with photocells, light sources with photocells, line-scan cameras, and two-dimensional array cameras. U.S. Pat. Nos. 3,740,563, 5,138,179 and 5,240,684, the entire disclosures of which are incorporated herein by reference, disclose methods and apparatus for determining the diameter of a crystal during the crystal growth process.
Unfortunately, conventional apparatus for automatically measuring crystal width are not sufficiently accurate or reliable for use during the different phases of crystal growth or for large diameter crystals in which the true maximum of the bright ring may be obscured from view by the solid body of the crystal itself. In an effort to correct this problem, conventional apparatus for measuring crystal width attempt to measure the meniscus at a chord or at a single point along the meniscus. However, such apparatus require precise mechanical positioning of the scanning device and are highly sensitive to fluctuations in melt level. Further, conventional measuring apparatus require frequent calibration by the operator of the crystal growing apparatus to ensure that the diameter remains within specification.
In addition to the problems described above, conventional apparatus for automatically measuring crystal diameter fail to provide accurate measurements when the crystal orbits, or moves in a pendular manner, as it is pulled from the melt. Known measurement apparatus are also unable to discriminate between the bright ring and reflections on the melt surface or on the growing crystal itself, resulting in unreliable measurements. Further, such apparatus are often unable to provide measurements when the viewport window is blocked by, for example, splashes of silicon.
Another disadvantage with conventional systems and methods for measuring crystal diameter is that they are unable to provide additional information regarding the crystal growth process, such as a measure of melt level and an indication of a loss of zero dislocation growth.
For these reasons, conventional apparatus fail to provide an accurate and reliable system of automatically determining crystal diameter for controlling the crystal growth process.